code: def test_empty_window_function() -> None:
      expr = (pl.col("VAL") / pl.col("VAL").sum()).over("KEY")
  
      df = pl.DataFrame(schema={"KEY": pl.String, "VAL": pl.Float64})
      df.select(expr)  # ComputeError
  
      lf = pl.DataFrame(
          schema={"KEY": pl.String, "VAL": pl.Float64}
      ).lazy()
      expected = pl.DataFrame(schema={"VAL": pl.Float64})
      assert_frame_equal(lf.select(expr).collect(), expected)
  
file_path: polars/py-polars/tests/unit/test_empty.py
imports: import pytest
  import polars
  from polars.testing import assert_frame_equal, assert_series_equal
label_desc: Use image node_id 5 for calling a global function with example usage: test_empty_window_function() without return types